Merriam-Webster's Collegiate Dictionary:usenetnounEtymology: probably from Usenix, an association of computer programmers using the operating system Unix (from users of Unix) + 1net (network)
Date: 1980
the aggregation of all the newsgroups on the Internet

Free On-line Dictionary of Computing:usenet

/yoos'net/ or /yooz'net/ (Or "Usenet news", from
"Users' Network") A distributed bulletin board system and
the people who post and read articles thereon. Originally
implemented in 1979 - 1980 by Steve Bellovin, Jim Ellis, Tom
Truscott, and Steve Daniel at Duke University, and supported
mainly by Unix machines, it swiftly grew to become
international in scope and, before the advent of the
World-Wide Web, probably the largest decentralised
information utility in existence.

Usenet encompasses government agencies, universities, high
schools, businesses of all sizes, and home computers of all
descriptions. In the beginning, not all Usenet hosts were on
the Internet. As of early 1993, it hosted over 1200
newsgroups ("groups" for short) and an average of 40
megabytes (the equivalent of several thousand paper pages) of
new technical articles, news, discussion, chatter, and
flamage every day. By November 1999, the number of groups
had grown to over 37,000.

To join in you originally needed a news reader program but
there are now several web gateways, cheifly Google Groups http://groups.google.com/ (originally Deja News). Some
web browsers include news readers and URLs beginning
"news:" refer to Usenet newsgroups.